Today, the serviceability of a transmission part is monitored by checking that the load is rotating or turning. Generally, this is monitored by means of a sensor and a trigger object. One of the sensor and the trigger object is mounted on the load while the other one is arranged on a stationary part in the surroundings of the load. The sensor detects the trigger object every time they pass each other, thus generating a signal that is proportional to the rotational or turning speed of the load. The sensors in such systems are usually magnetic sensors, optical sensors, mechanical sensors, etc.
A drawback of such monitoring systems is that a portion of the system has to be mounted on the load, thereby possibly affecting properties of the load. Other drawbacks of mounting a portion of the monitoring system on the load are that the portion may fall off, that the sensing/signalling properties of the portion may be affected by dirt, dust, or other obstacles, and that the sensor and trigger object have to be very carefully mounted in order to ensure acceptable function.
Another drawback of such monitoring systems is that service personnel is required to visit the site of the system even when the malfunction originates from disturbances or malfunctions in the monitoring system.
In SU 1 992 268 a belt tension control system for a circular saw drive is described. The belt tension control utilises the fact that increased motor speed reduces the stator current of the motor. Thus, when the belt tension is low, stator current falls sharply and releases a contact. This belt tension control system presents the drawback of requiring service personnel visiting the site of the system even if the detected fault only results from a slipping transmission belt.
In SU 1 666 413 there is described a belt-type conveyor control with breakdown prevention. The system excludes emergency modes resulting from slippage of driving drums driving a conveyor belt. In order to achieve this, the moment variation rate of an asynchronous motor is measured using current sensors. The maximum variation rate is then used to specify a non-sensitivity zone. Even if this system is more reliable than the previously mentioned, the system still results in a rather high percentage of false failure alarms. The high percentage of false failure alarms is particularly evident in systems in which the torque requirement of the load varies over time.